It is known to provide a planetary friction drive in which the planet wheels roll upon a conical portion of a drive shift and roll in contact with a stationary ring surface. The planet wheels are formed with conical peripheries or peripheral surfaces and have axes which are fixedly positioned with respect to a planet carrier. The planet carrier or entrainment plate is generally resiliently formed in the regions in which the planet wheels are mounted.
It is important with such constructions that the generatrices of the peripheral surfaces of all of the planet wheels and the axes of all of the planet wheels intersect at a common point along the longitudinal axis of the drive shaft since only in this case is a uniform pressure applied by the planet wheels to the cone of the shaft and vice versa. In addition, the conical surfaces of the ring against which the planet wheels roll must also be so formed that their generatrices coincide with the generatrices of the conical surfaces of the planet wheels at the region of which the planet wheels engage the stationary conical surfaces. The distance between the conical surface of the shaft and the conical surface of the planet wheel must also correspond exactly to the diameter of the planet wheels.
Because of the somewhat resilient character of the planet wheel carrier it is possible that the axis of one or another of the planet wheels will tilt out of its original or intended position so that the coincidence of the generatrices between the conical surfaces will no longer be maintained.
This will give rise to a so-called point contact between the planet wheels and the surfaces against which the planet wheels roll with correspondingly high wear in the limited contact region and binding and distortion within the transmission.
Furthermore, the very nature of the construction prevents miniaturization beyond a certain point because fabrication tolerances in general cannot be lowered without increasing play and binding forces within the transmission.